Can I install drain valves in piping at Backflow Preventer?

I have a Febco 765 1 in. PVB Backflow Preventer, and I can turn the in ground main sprinkler valve off in the winter but I assume that water stays in the vertical 1" PVC lines and could freeze. I'm thinking about installing Tee's and drain valves right at ground level on both PVC line so I can drain them in the winter.